1. Field of the Invention
This invention relates to ultrasonic diagnostic imaging, and more particularly, to an imaging system beamformer having a cascade structure that provides improved phase aberration delay correction.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic transducer is often packaged within a handheld device that allows the physician substantial freedom to manipulate the transducer easily over a desired area of interest. The transducer can then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal (VDT). The image information may also be stored to enable other physicians to view the diagnostic images at a later date.
In one particular method of ultrasonic imaging, referred to as Phased Array Sector Scanning (PASS), the transducer comprises an array of piezoelectric elements that are individually driven by separate electrical signals. By controlling the phase and amplitude of the signals, the ultrasonic wave produced by the piezoelectric elements can be focused, or steered, to a single point. The received echoes from the individual ultrasonic waves are then summed together and processed in a manner that yields a net signal characterizing the single point, a process referred to as beamforming. The imaging operation can be repeated to collect information from a series of points along a scan line. A plurality of such scan lines would provide a sector scan of an entire region of interest.
An important assumption of the beamforming process is that the acoustic velocity within the human tissues is a constant (generally a value of 1,540 meters per second (m/s) is used). In reality, however, the acoustic velocity varies substantially since the human body is composed of inhomogeneous layers of different tissue types, such as subcutaneous fat, muscle and bone. Moreover, the tissue boundaries are substantially non-uniform, having ridges and bumps of varying thicknesses, densities, and acoustic velocities. For example, the propagation velocity of an ultrasonic wave varies from approximately 1,470 m/s in fat, to greater than 1,600 m/s in muscle and nervous tissue, to as much as 3,700 m/s in bone. These aberrations in the tissues slow down or speed up the acoustic waves emanating from certain elements in the transducer array such that the signals corresponding to the delayed waves do not have the desired phase when summed with the other signals during beamforming. As a result of this phase aberration, various types of ultrasound image anomalies can be experienced, including image artifacts, range shifts, geometric distortions, broadening of the transducer beam pattern which degrades lateral resolution, and increased side lobes which reduce the contrast resolution of the image. These various anomaly types tend to be especially prevalent and degrading in certain tissue imaging operations in which a relatively large degree of inhomogeneities exist, such as abdominal, transcranial and breast imaging.
Various methods have been proposed to correct for the undesirable phase aberration. One such method is disclosed in U.S. Pat. No. 5,172,343, to O'Donnell for ABERRATION CORRECTION USING BEAM DATA FROM A PHASED ARRAY ULTRASONIC SCANNER. O'Donnell discloses a system for phase aberration correction in which the phase delay error between each transducer element and the nearest adjacent transducer element is estimated by cross-correlating the signals from these two elements. A correction delay is supplied to each particular element based on a sum total of all the estimated delays between that element and a reference element (such as the first element of the array).
A significant drawback of the O'Donnell system is that dead or weak transducer elements tend to degrade performance of the entire system, since errors in phase estimation are accumulated across all the elements of the array. This accumulation of errors ultimately causes inaccuracies in the signal phase profile. It is possible to detect the dead or weak elements and remove their deleterious effects, or to mitigate the overall accumulation of errors by iteratively defining the phase profile over several consecutive pulse repetitions. Nevertheless, these corrective measures substantially increase the magnitude of signal processing with a resulting decrease in imaging speed.
Another phase aberration correction system is disclosed in U.S. Pat. No. 5,331,964, to Trahey et al. for ULTRASONIC PHASED ARRAY IMAGING SYSTEM WITH HIGH SPEED ADAPTIVE PROCESSING USING SELECTED ELEMENTS. Trahey discloses a system in which the phase error of a particular element (or group of elements) is estimated by maximizing the brightness of a signal formed by adding the signal from this element to a signal from a reference element (or group of elements). While this technique tends to be more robust in terms of differentiating noise and weak signals, it requires a complex and thus expensive parallel implementation in order to provide commercially acceptable imaging speed. Also, this technique requires that some acoustic signals be used to measure the phase aberration, and other acoustic signals be used to form the image, which tends to decrease the imaging speed.
Accordingly, a critical need exists for a phase aberration correction method and apparatus for use with an ultrasonic phased array sector scanning system that is capable of overcoming these deficiencies of the prior art. Specifically, such a phase aberration correction method and apparatus should be able to limit the accumulative effect of localized defects or errors within the transducer array, and should be capable of parallel implementation with minimal complexity or impact upon imaging speed.